tavshed

Download image from raspberrypi.org

Plug in SD card

Identify SD card
df -h

Note: /dev/disk1s1 is the disk on my MacBook Pro. You need to review and change the disk info as necessary.

Unmount
sudo diskutil unmount /dev/disk1s1

Apply image to SD card
sudo dd bs=1m if=2012-09-18-wheezy-raspbian.img of=/dev/rdisk1